The activity of the enzyme rhodanese was measured during the embryonic development of liver and kidney in the mouse. The organ-specific nature of ribosomes was established using SDS-acrylamide gel electrophoresis. The organ-specific differences in the individual ribosomal proteins was related to the development of the embryonic liver and kidney. The nucleotide guanosine tetraphosphate (ppGpp) was detected in early organ rudiments prior to specialized protein syntheis, and is considered to act as a regulator during organogenesis. The postnatal increase and biochemical activity of microbodies was related to the functional differentiation of the proximal kidney tubule. A sensitive radioassay for glycerol kinase activity during differentiation of the kidney was developed. The developmental profile for this enzyme will be measured. An extract was prepared from embryonic brain with possible tubulogenic activity and is being characterized.